drm/i915/vga: Get rid of intel_vga_reset_io_mem()

Remove the MSR VGA register access from the power well hook, and
just do it once in intel_vga_disable().

Turns out that the hardware has two levels of MDA vs. CGA decode
logic: GPU level and display engine level. When we write the MSR
register MDA/CGA mode selection bit both decode logics are updated
accordingly, so that whichever register accessed the GPU claims
will also be claimed by the display engine on the RMbus. If the
two get out of sync the GPU will claim the register accesses but
the display engine will not, leading to RMbus NoClaim errors.

The way to get the two decode logics out of sync is by resetting
the power well housing the VGA stuff, while we are in CGA mode.
At that point only the display engine level decode logic will be
updated with the new MSR register reset value (which selects MDA
mode), and the GPU level decode logic will retain its previous
state (GGA mode). To avoid the mismatch we just have to switch
to MDA mode with an explicit MSR register write.

Currently this is being done in a somewhat dodgy manner whenever
the power well gets enabled. But doing if from the power well
hook is not actually necessary since the GPU level decode logic
will retain its state even when the power well is disabled. Ie.
doing it just the one time is sufficient, and that can be done
when we're anyway writing other VGA registers while disabling
the VGA plane.

See commit f9dcb0dfee98 ("drm/i915: touch VGA MSR after we
enable the power well") for the original details.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251208182637.334-3-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_display_power_well.c b/drivers/gpu/drm/i915/display/intel_display_power_well.c
index db185a8591339d9924c3ab55feaf2b274b6495d5..52b20118ace6c69b83567c8339041a89b60b8716 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_display_power_well.c
+++ b/drivers/gpu/drm/i915/display/intel_display_power_well.c
@@ -204,9 +204,6 @@ int intel_power_well_refcount(struct i915_power_well *power_well)
 static void hsw_power_well_post_enable(struct intel_display *display,
                                       u8 irq_pipe_mask, bool has_vga)
 {
-       if (has_vga)
-               intel_vga_reset_io_mem(display);
-
        if (irq_pipe_mask)
                gen8_irq_power_well_post_enable(display, irq_pipe_mask);
 }

diff --git a/drivers/gpu/drm/i915/display/intel_vga.c b/drivers/gpu/drm/i915/display/intel_vga.c
index f13734cfd90424459760338983b8554a4529a654..39c68aec647b8e7ff71f3cc7b0b974c31a827644 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_vga.c
+++ b/drivers/gpu/drm/i915/display/intel_vga.c
@@ -47,8 +47,8 @@ void intel_vga_disable(struct intel_display *display)
        struct pci_dev *pdev = to_pci_dev(display->drm->dev);
        i915_reg_t vga_reg = intel_vga_cntrl_reg(display);
        enum pipe pipe;
+       u8 msr, sr1;
        u32 tmp;
-       u8 sr1;
 
        tmp = intel_de_read(display, vga_reg);
        if (tmp & VGA_DISP_DISABLE)
@@ -66,35 +66,35 @@ void intel_vga_disable(struct intel_display *display)
 
        /* WaEnableVGAAccessThroughIOPort:ctg,elk,ilk,snb,ivb,vlv,hsw */
        vga_get_uninterruptible(pdev, VGA_RSRC_LEGACY_IO);
+
        outb(0x01, VGA_SEQ_I);
        sr1 = inb(VGA_SEQ_D);
        outb(sr1 | VGA_SR01_SCREEN_OFF, VGA_SEQ_D);
+
+       msr = inb(VGA_MIS_R);
+       /*
+        * VGA_MIS_COLOR controls both GPU level and display engine level
+        * MDA vs. CGA decode logic. But when the register gets reset
+        * (reset value has VGA_MIS_COLOR=0) by the power well, only the
+        * display engine level decode logic gets notified.
+        *
+        * Switch to MDA mode to make sure the GPU level decode logic will
+        * be in sync with the display engine level decode logic after the
+        * power well has been reset. Otherwise the GPU will claim CGA
+        * register accesses but the display engine will not, causing
+        * RMbus NoClaim errors.
+        */
+       msr &= ~VGA_MIS_COLOR;
+       outb(msr, VGA_MIS_W);
+
        vga_put(pdev, VGA_RSRC_LEGACY_IO);
+
        udelay(300);
 
        intel_de_write(display, vga_reg, VGA_DISP_DISABLE);
        intel_de_posting_read(display, vga_reg);
 }
 
-void intel_vga_reset_io_mem(struct intel_display *display)
-{
-       struct pci_dev *pdev = to_pci_dev(display->drm->dev);
-
-       /*
-        * After we re-enable the power well, if we touch VGA register 0x3d5
-        * we'll get unclaimed register interrupts. This stops after we write
-        * anything to the VGA MSR register. The vgacon module uses this
-        * register all the time, so if we unbind our driver and, as a
-        * consequence, bind vgacon, we'll get stuck in an infinite loop at
-        * console_unlock(). So make here we touch the VGA MSR register, making
-        * sure vgacon can keep working normally without triggering interrupts
-        * and error messages.
-        */
-       vga_get_uninterruptible(pdev, VGA_RSRC_LEGACY_IO);
-       outb(inb(VGA_MIS_R), VGA_MIS_W);
-       vga_put(pdev, VGA_RSRC_LEGACY_IO);
-}
-
 static int intel_gmch_vga_set_state(struct intel_display *display, bool enable_decode)
 {
        struct pci_dev *pdev = to_pci_dev(display->drm->dev);